Guides

Coating Thickness Gauges

Coating thickness gauges are specialized instruments for measuring coatings on metal objects. They are divided into 3 groups. Ferrous measure coating/paint/galvanizing on ferrous steel substrates. Non Ferrous measure coating / anodizing on non-ferrous substrates (e.g. Aluminum). Finally, Ferrous & Non Ferrous, which cover both cases, are widely used for checking car paint, in metal structures, etc.

An excitation coil wound around an iron core, when a low-frequency alternating current is applied to its ends, creates an alternating magnetic field in the air gap around its poles. This arrangement is used to measure the thickness of a coating surface according to the magnetic induction method.

Figure 1: Typical applications for surface thickness gauges using the principles of Magnetic Induction or Eddy current.

When a pole approaches a piece of iron, the magnetic field will be strengthened. It then generates a voltage in a second coil (measuring coil). This voltage depends on the distance from the iron. By placing the iron core or the measuring sensor on the coating, a defined distance results between the pole and the iron and therefore a defined coil voltage. This is evaluated electronically and displayed digitally as coating thickness.

Figure 2: The operating principle of magnetic induction on an insulating coating with a ferromagnetic substrate.Ferrous

Operating principle of Eddy eddy current on a Non- Ferrous substrate

Measurement according to the Eddy current method uses only one coil through which a high-frequency alternating current flows. This produces an electromagnetic alternating field around the coil. When the coil with the alternating field approaches a non-ferrous metal, a varying current: called eddy current: is induced in that metal. This produces another electromagnetic alternating field in the opposite direction, weakening the original field. The effect on the coil is shown by the change in inductance, a characteristic property of each coil. By placing the measuring sensor on the coating, a defined distance results between the coil and the non-ferrous metal, thus leading to a defined inductance value in the coil. This will be evaluated electronically and displayed digitally as coating thickness.

Figure 3: The operating principle of Eddy current on an insulating coating with a non-magnetic substrate.Non-Ferrous

A device that is compatible with both types of applications Ferrous & Non-Ferrous

Coating thickness gauges that operate in both of the above cases have a combination of F and N type sensors, as well as both coil systems mounted in the measuring sensor so that the alternating field of both coils is affected: obtaining results in both low-frequency and high-frequency alternating fields: when the sensor is placed on the surface to be measured. The measurement signals of the two coils are automatically evaluated electronically by a microprocessor, so that it can select the measurement process corresponding to the substrate material and display the correct layer thickness, regardless of whether the measurement takes place on iron or non-ferrous metal.